Emulsion polymerization is the most important industrial method for manufacture of aqueous dispersion of polymers. Emulsion polymerization is typically performed in an aqueous medium in the presence of a surfactant and a water soluble initiator and is usually rapidly producing high molecular weight homo or copolymers at high solids content and low dispersion viscosity. Its application requires the emulsification of the monomer in a medium, usually water, through the use of emulsifiers. These are supplied in addition to the other ingredients that go into most polymerizations, such as the initiator and chain transfer agents. The use and type of emulsifier determines many of the characteristics of the produced polymer or copolymer, which is typically a latex (stable colloidal suspension of polymer particles in a continuous phase, usually water). Moreover, the emulsifier usually cannot be completely removed from the latex. For this reason, and because of the great unpredictability of the efficacy of a given surface-active agent as an emulsifier in polymerization, many compounds that would theoretically be useful are not.
It is also known that emulsion polymerization requires the use of a surfactant to form a stable emulsion of monomers and to prevent coagulation of the product polymer. Surfactants are generally categorized into two types: either non-polymerizable, or polymerizable, that is co-polymerizable with the monomers for polymer formation. A problem which has arisen with the use of non-polymerizable surfactants is that they remain as a residue in the product polymer and, as they can be extracted by water, they make the product sensitive to water. Surfactants are also categorized as anionic, cationic, non-ionic or zwitterionic depending on their chemical makeup.
With the customary emulsion polymerization processes, suitable latices have been difficult to obtain since the latices usually contain particles of varying size and are either too fine or too large. Various proposals have heretofore been made to overcome these difficulties but not with the ultimate success desired. For example, the use of various different emulsifiers and catalysts have been proposed. Also, varying the conditions of polymerization has been suggested. However, in most of these cases, too much coagulation occurred with the resulting latex containing too much coagulum or partially agglomerated particles which precipitate reducing the yield. Further, the shelf life of such latices leave much to be desired. It is desirable to have latices which change very little during storage with respect to viscosity and have and maintain good heat stability.
The final product resulting from emulsion polymerization is normally an opaque, grey or milky-white dispersion of high molecular weight polymer(s) at a solids content of typically 30-60% in water. Said dispersion typically comprises acrylic, methacrylic and crotonic acid homo and copolymers, methacrylate and acrylate ester homo or copolymers, vinyl acetate homo or copolymers, vinyl and vinylidene chloride homo or copolymers, ethylene homo or copolymers, styrene and butadiene homo or copolymers, acrylamide homo or copolymers, butadiene-acrylonitrile copolymers, styrene-acrolein copolymers and/or where applicable carboxylated versions. Traditional applications for such aqueous dispersions are adhesives, binders for fibres and particulate matter, protective and decorative coatings, dipped goods, foam, paper coatings, backings for carpet and upholstery, modifiers for bitumens and concrete and thread and textile modifiers. More recent applications include biomedical applications as protein immobilisers, visual detectors in immunoassays, as release agents, in electronic applications as photoresists for circuit boards, in batteries, conductive paint, copy machines and as key components in molecular electronic devices.
Ethoxylated Styrenated Phenols have been widely disclosed as effective and efficient pigment dispersants in a variety of applications. U.S. Pat. No. 6,736,892 (2004) discloses anionic Sytrenated Phenol Ethoxylates as pigment dispersants in water based ink and coating applications. U.S. application 0235877 A1 (2005) discloses fatty acid esters of Styrenated Phenol Alkoxylates as effective and efficient pigment dispersants for solvent based systems. U.S. Pat. No. 5,035,785 (1991) discloses nonionic Styrenated Phenol Ethoxylates as effective and efficient pigment dispersants in electrodeposition baths. The dispersants were also found to increase film build and dramatically improve film appearance and maintain the improved film appearance in this application. Thus the utility of Styrenated Phenol Ethoxylates, both anionic and nonionic, in separating and stabilizing pigment particles against aggregation has been well known and demonstrated in both aqueous and non-aqueous systems. In these applications, these surfactants are added as part of the formulation comprising the resin used as a binder or film former, the pigment paste or dispersion, and other additives such as coalescing aids, viscosity modifiers, and other additives well known to those familiar with the art.
In emulsion polymerization of ethylenically unsaturated monomers, it is well known to those familiar with the art that surfactants are necessary and essential ingredients required for the polymerization reaction to occur in the aqueous phase. It is also well known to those familiar with the art that surfactants further function by stabilizing the latex particles against aggregation from shear or mechanical force and also stabilize the latex particles from aggregation due to the addition of electrolyte to the latex. In the emulsion polymerization process, since the radical polymerization takes place inside the surfactant formed micelles, the surfactants in emulsion polymerization are essential components in the manufacture of the latex. Benjamin B. Kline and George H. Redlich, “The Role of Surfactants in Emulsion Polymerization”, Surfactant Science Series, 26, 1988) It is also well known to those skilled in the art that the selection of surfactant type and level is also the determining factor in many emulsion properties, in particular the particle size of the latex particles. Low particle size is highly desirable in pigmentation and results in higher gloss in the final film or coating.
We have now found that the Styrenated Phenol Ethoxylates are useful as surfactants in the emulsion polymerization of ethylenically unsaturated monomers. The use of both anionic and nonionic Styrenated Phenol Ethoxylates in the emulsion polymerization process provides latexes with small particle size and small particle size distributions. Furthermore, latexes prepared using Styrenated Phenol Ethoxylates have excellent mechanical and chemical stability. The Styrenated Phenol Ethoxylates of the present invention may also be used in combination with conventional surfactants to improve latex properties.